**2. Basic concepts and regulations**

#### **2.1 Characteristics of the base gas**

The term base gas refers to the natural gas provided by the gas utilities (GUs) in the respective coverage areas without the addition of biogenic gas. The classification of the various natural gases distributed in Germany into H and L gases is made according to worksheet DVGW-G 260 (German Association of Gas and Water [DVGW], 2008). The Wobbe Index, which is a measure of the thermal energy released on the burner of a gas appliance or the energy transported through a pipe has a special significance here.

The Wobbe Index is an important variable to assess the interchangeability of fuel gases. When replacing one fuel gas with another, the output of the burner changes in proportion to the ratio of the Wobbe index. Its definition according G 260 is given in equation 1:

$$\mathcal{W}\_{S,n} = \frac{H\_{S,n}}{\sqrt{d}} \; \; 1 \; \; d = \frac{\rho\_{\text{Gas},n}}{\rho\_{\text{Luft},n}} \; \text{With the relative density and the calorific value } \; H\_{S,n} \tag{1}$$

The upper value of the Wobbe index total range should not be exceeded. A shortfall in the lower value is acceptable under certain conditions and subject to a time limit. Both limits are specified in the German regulations. The nominal value listed in G 260 is used for setting the gas appliances used. Technically, the local variation range could be omitted, since the gas appliances are set to a nominal value. Currently however there are still many appliances set to differing values. The major boundary conditions are dictated by the Wobbe Index total range, the calorific range and the relative density, since conditioning with air and / or liquefied gas influences precisely these variables.

For the calorific value of a gas mixture, equation 2 states:

$$H\_{s,n} = \sum\_{i} r\_i H\_{s,n,i} \quad \text{or} \quad \rho\_n = \sum\_{i} r\_i \rho\_{n,i} \, ^2 \tag{2}$$

<sup>1</sup> Indexing "S" (superior) is the formation with calorific value and "n" standard conditions

<sup>2</sup> ri denotes the volume fraction of component i

Schematic recommendations include the answers to the key questions listed below and allow a simple "read out" of the target properties, taking into account the current regulatory requirements. Gas utilities (GU) and operators can already in the planning phase determine the feed options and requirements with the help of the graphs. The key questions are:

1. What qualities and technical characteristics of combustion must processed biogas have at the very least so that it can be fed into grids in which the natural gases typical in Germany are present as base gases, without having to make changes to the grid? 2. What additional aspects need to be considered when feeding processed biogas into the existing natural gas grid, taking into account fairness in the billing process, properties

The following conditions are to be observed in addition to point 1 : Engine applications, natural gas filling stations (methane, K number, condensation of higher hydrocarbons) and

The term base gas refers to the natural gas provided by the gas utilities (GUs) in the respective coverage areas without the addition of biogenic gas. The classification of the various natural gases distributed in Germany into H and L gases is made according to worksheet DVGW-G 260 (German Association of Gas and Water [DVGW], 2008). The Wobbe Index, which is a measure of the thermal energy released on the burner of a gas

The Wobbe Index is an important variable to assess the interchangeability of fuel gases. When replacing one fuel gas with another, the output of the burner changes in proportion to

The upper value of the Wobbe index total range should not be exceeded. A shortfall in the lower value is acceptable under certain conditions and subject to a time limit. Both limits are specified in the German regulations. The nominal value listed in G 260 is used for setting the gas appliances used. Technically, the local variation range could be omitted, since the gas appliances are set to a nominal value. Currently however there are still many appliances set to differing values. The major boundary conditions are dictated by the Wobbe Index total range, the calorific range and the relative density, since conditioning with air and / or

*H rH* <sup>=</sup> or *n i ni*,

ρ

<sup>=</sup> With the relative density and the calorific value *HS n*, (1)

*i*

 ρ

<sup>=</sup> *<sup>r</sup>* 2 (2)

appliance or the energy transported through a pipe has a special significance here.

the ratio of the Wobbe index. Its definition according G 260 is given in equation 1:

(functionality of end-user equipment) and cost effectiveness?

industrial customers.

**2. Basic concepts and regulations 2.1 Characteristics of the base gas** 

,

*<sup>d</sup>* <sup>=</sup> <sup>1</sup> ,

*d* ρ

, *Gas n Luft n*

ρ

liquefied gas influences precisely these variables.

2 ri denotes the volume fraction of component i

For the calorific value of a gas mixture, equation 2 states:

*sn i sni* , , , *i*

1 Indexing "S" (superior) is the formation with calorific value and "n" standard conditions

*S n*

*H*

,

*S n*

*W*

When considering equation 2 and table 1, it is clear that even small volumes of higher hydrocarbons affect the parameters of combustion of the gas mixture, due to the greater density and calorific values. The same applies to air and carbon dioxide.


Table 1. Standard density and calorific value of the main components

In addition to the basic requirements for the gas properties, limits for accompanying substances are specified in worksheet G 260, which may not be exceeded.


Table 2. Permitted substances in the gas according to DVGW worksheet G 260 (DVGW, 2008)

Important boundary conditions are determined by the dew points, the oxygen content and the sulphur content. Information on the dew points is formulated so that condensation can be excluded. As far as the oxygen content is concerned, the grids in Germany can be regarded as dry and therefore the limit of 3 vol -% is to be applied. It should be noted at this point that at the long-range transport level significantly lower O2contents, usually in the low ppm range are to be observed (EASEE gas, CPB European Association for the Streamlining of Energy Exchange-gas Common Business Practice, 2005) for cross-border distribution (H gas).

The raw gas must be cleaned, processed (according to G 260) and compressed to the pressure of the grid operator. Under no circumstances should health risks arise from processed gas. For injection into the distribution network of a local GU, the gas must be

Conditioning of Biogas for Injection into the Natural Gas Grid 373

according to the regional location of the customer. The gas delivered to customers may not deviate in calorific value by more than 2% from the calorific value for billing purposes. To check this, the mean values and the quantity-weighted average in the downstream network

Since worksheet G 486 is particularly applicable due to the admixing of propane and butane

**2.3 The worksheet G 486 "Gas quantity measurement, compressibility factors and gas law deviation factors of natural gases, calculation and application" from the DVGW** 

The determination of gas quantity, or volume is carried out under operating conditions (metering conditions). The result is an operational flow VB (TB, PB) as a function of temperature and pressure. This operational flow needs to be converted to standard conditions (TN = 0 ° C, pN = 1.01325 bar) in order to compare volumes and so that it can be used as an input for gas billing. Since the model for an ideal gas is only approximately valid for real gases at low pressures, a compressibility factor Z (T, p, xi) is introduced into the equation of state for ideal gases. The compressibility factor is mathematically approximated by a series expansion of the molar density (virial approach). The calculation of standard

> *B N*

( , ) (3)

*Z Z*

*B N N B*

*T p T p*

Two methods for calculating compressibility factors are given in G 486 including the supplementary sheets: The standard GERG-88 virial equation and the AGA8-DC92 equation of state. The former requires input parameters of p, T, HS, N, ρ, xCO2 and XH2, the latter the mole fractions. The AGA8 equation of state requires a full analysis by means of a process

The term liquid gas (Liquefied Petroleum Gas3 it refers to C3 and C4 hydrocarbons or mixtures thereof. It is generated as a by-product in petroleum refining and as an associated gas from the extraction of oil and natural gas. LPG is gaseous at room temperature under atmospheric conditions, but can be liquefied at low pressures. In liquid form, its specific volume is about 260 times smaller than in the gaseous state. Therefore, large amounts of

The transportation of LPG is carried out worldwide by tanker ships, barges, pipelines, by rail tank cars, road tankers or in liquefied gas cylinders. LPG is stored in stationary tank facilities or in gas cylinders. Up to a tank size of 2.9 t capacity, the above-ground installation does not require a permit. From a tank capacity of 2.9 tonnes, the federal emission

<sup>3</sup> The term LPG is not to be understood as "car gas" which has a different propane / butane mixture ratio

*B B N N*

*V T p <sup>V</sup> <sup>T</sup> <sup>p</sup>* <sup>=</sup> ( , )

The ratio of the compressibility factors is called the gas law deviation factor.

**2.4 Conditioning with liquid gas (propane / butane)** 

energy can be transported and stored in relatively small containers.

as considered in this report, essential aspects are explained below.

are to be determined.

volume is thus given by equation 8. 3:

gas chromatograph.

**regulations** 

odorized according to G 280-1(DVGW, 2004). In addition, the presence of certain gas accompanying substances such as H2S must be monitored regularly. Furthermore, for a time and heat equivalent transfer, the calorific value for billing purposes must also be known.

After processing the raw gases for the public gas supply, these can be used according to G 260 as an exchange gas (G 260 Section 4.4.2) or as additional gas (G 260 4.2, 4.3), (gas for conditioning) and be made available to the grid operator at the transfer interface. (Note: it should be noted that additional gas feeds are only possible in a single pipeline under certain circumstances.)

Put simply, it can be said that the conditions given in worksheets G 260 and G 262 (DVGW, 2007) ensure that the customers' appliances will work correctly. Sensitive industrial processes sometimes require much tighter limits on the gas properties (e.g. glass and ceramics production). The DVGW worksheet G 260 is very often a component of supply contracts and is an expression of the flexibility of the gas sector, which is necessary in the procurement of natural gas, in order to deliver natural gas from various gas fields into the transport and distribution system of the German gas industry and on to the customer. Due to its geographical location, historical and political development, in Germany natural gases from the most diverse of foreign origins as well as natural gases from its own sources are thus forwarded to the customers, with the guarantee of security of supply, functionality of the natural gas applications and fair billing.

In summary, in order to inject biogas into the natural gas grid, the above requirements must be met. In addition to excluding the gas accompanying substances by cleaning and processing the biogas, further conditioning to adjust the Wobbe Index and the calorific value to the target grid is required, depending upon the case in point.

The processed, conditioned biogas is considered to be an exchange gas, if it meets the requirements set out in G 260, G 262 and G 685. Furthermore, during conditioning with liquid gas, limits according to G 486 (DVGW, 1992) need also to be considered.

### **2.2 "Gas billing" according to worksheet G 685 of the DVGW regulations (DVGW, 2008)**

For billing, two parameters need to be determined: The volume flow of the fuel gas under standard conditions (T = 0 ° C, p = 1.01325 bar) and the calorific value for billing purposes. The determination of the standard volume flow from the operational flow is done using the procedure described in G 685, taking into account the temperature and atmospheric pressure. At pressures greater than 1 bar, real gas behaviour should be taken into account (G 486). The calorific value for billing purposes is determined from the calorific values of the feed for each billing interval (such as 1 month (special contract customers) or 1 year (residential customers)) in a coverage area (the total area, which is supplied by the GU, not necessarily contiguous). If the calorific values of the feed change over time, then these are determined arithmetically or by volume-weighted methods over the month.

If gases with different calorific values are distributed, then the following, according to G 685 (DVGW, 2008), applies:

If gases are fed through geographically separate feed points into a grid or into gas supply areas which cannot be isolated, the calorific value for billing purposes is to be determined

odorized according to G 280-1(DVGW, 2004). In addition, the presence of certain gas accompanying substances such as H2S must be monitored regularly. Furthermore, for a time and heat equivalent transfer, the calorific value for billing purposes must also be known.

After processing the raw gases for the public gas supply, these can be used according to G 260 as an exchange gas (G 260 Section 4.4.2) or as additional gas (G 260 4.2, 4.3), (gas for conditioning) and be made available to the grid operator at the transfer interface. (Note: it should be noted that additional gas feeds are only possible in a single pipeline under certain

Put simply, it can be said that the conditions given in worksheets G 260 and G 262 (DVGW, 2007) ensure that the customers' appliances will work correctly. Sensitive industrial processes sometimes require much tighter limits on the gas properties (e.g. glass and ceramics production). The DVGW worksheet G 260 is very often a component of supply contracts and is an expression of the flexibility of the gas sector, which is necessary in the procurement of natural gas, in order to deliver natural gas from various gas fields into the transport and distribution system of the German gas industry and on to the customer. Due to its geographical location, historical and political development, in Germany natural gases from the most diverse of foreign origins as well as natural gases from its own sources are thus forwarded to the customers, with the guarantee of security of supply, functionality of

In summary, in order to inject biogas into the natural gas grid, the above requirements must be met. In addition to excluding the gas accompanying substances by cleaning and processing the biogas, further conditioning to adjust the Wobbe Index and the calorific value

The processed, conditioned biogas is considered to be an exchange gas, if it meets the requirements set out in G 260, G 262 and G 685. Furthermore, during conditioning with

For billing, two parameters need to be determined: The volume flow of the fuel gas under standard conditions (T = 0 ° C, p = 1.01325 bar) and the calorific value for billing purposes. The determination of the standard volume flow from the operational flow is done using the procedure described in G 685, taking into account the temperature and atmospheric pressure. At pressures greater than 1 bar, real gas behaviour should be taken into account (G 486). The calorific value for billing purposes is determined from the calorific values of the feed for each billing interval (such as 1 month (special contract customers) or 1 year (residential customers)) in a coverage area (the total area, which is supplied by the GU, not necessarily contiguous). If the calorific values of the feed change over time, then these are

If gases with different calorific values are distributed, then the following, according to G 685

If gases are fed through geographically separate feed points into a grid or into gas supply areas which cannot be isolated, the calorific value for billing purposes is to be determined

liquid gas, limits according to G 486 (DVGW, 1992) need also to be considered.

**2.2 "Gas billing" according to worksheet G 685 of the DVGW regulations** 

determined arithmetically or by volume-weighted methods over the month.

circumstances.)

**(DVGW, 2008)** 

(DVGW, 2008), applies:

the natural gas applications and fair billing.

to the target grid is required, depending upon the case in point.

according to the regional location of the customer. The gas delivered to customers may not deviate in calorific value by more than 2% from the calorific value for billing purposes. To check this, the mean values and the quantity-weighted average in the downstream network are to be determined.

Since worksheet G 486 is particularly applicable due to the admixing of propane and butane as considered in this report, essential aspects are explained below.

#### **2.3 The worksheet G 486 "Gas quantity measurement, compressibility factors and gas law deviation factors of natural gases, calculation and application" from the DVGW regulations**

The determination of gas quantity, or volume is carried out under operating conditions (metering conditions). The result is an operational flow VB (TB, PB) as a function of temperature and pressure. This operational flow needs to be converted to standard conditions (TN = 0 ° C, pN = 1.01325 bar) in order to compare volumes and so that it can be used as an input for gas billing. Since the model for an ideal gas is only approximately valid for real gases at low pressures, a compressibility factor Z (T, p, xi) is introduced into the equation of state for ideal gases. The compressibility factor is mathematically approximated by a series expansion of the molar density (virial approach). The calculation of standard volume is thus given by equation 8. 3:

$$\frac{V(T\_N, p\_N)}{V(T\_B, p\_B)} = \frac{T\_N p\_B}{T\_B p\_N} \frac{Z\_N}{Z\_B} \tag{3}$$

The ratio of the compressibility factors is called the gas law deviation factor.

Two methods for calculating compressibility factors are given in G 486 including the supplementary sheets: The standard GERG-88 virial equation and the AGA8-DC92 equation of state. The former requires input parameters of p, T, HS, N, ρ, xCO2 and XH2, the latter the mole fractions. The AGA8 equation of state requires a full analysis by means of a process gas chromatograph.

## **2.4 Conditioning with liquid gas (propane / butane)**

The term liquid gas (Liquefied Petroleum Gas3 it refers to C3 and C4 hydrocarbons or mixtures thereof. It is generated as a by-product in petroleum refining and as an associated gas from the extraction of oil and natural gas. LPG is gaseous at room temperature under atmospheric conditions, but can be liquefied at low pressures. In liquid form, its specific volume is about 260 times smaller than in the gaseous state. Therefore, large amounts of energy can be transported and stored in relatively small containers.

The transportation of LPG is carried out worldwide by tanker ships, barges, pipelines, by rail tank cars, road tankers or in liquefied gas cylinders. LPG is stored in stationary tank facilities or in gas cylinders. Up to a tank size of 2.9 t capacity, the above-ground installation does not require a permit. From a tank capacity of 2.9 tonnes, the federal emission

<sup>3</sup> The term LPG is not to be understood as "car gas" which has a different propane / butane mixture ratio

Conditioning of Biogas for Injection into the Natural Gas Grid 375

cases are described in the DVGW study "Developing a scientific basis for injecting biogas

First, the most important combustion-related characteristic data are listed for the selected base gases, in order to summarise the requirements imposed on the biogas, in particular

Then the characteristics of the processed biogas with a methane content of 94 - 99,5 Vol.-% will be compiled in order to determine the conditioning necessary to adapt to the base gas.

To attain the Wobbe index of an L gas, air must be added, which lowers the calorific value. In the case of L gases with higher calorific values, liquid gas needs to be added. When higher demands are placed on the calorific value (H gases), a liquid gas addition is necessary. A processed biogas containing 99.5 Vol -% methane has a calorific value of 11.0 kWh / m³.

For the calculations of the compositions in the following sections, the values from the following table 3 will be used. The composition of air is taken to be 20.95 Vol.-% oxygen and

CH4 11,064 22,36 CO2 0 22,261 N2 0 22,403 O2 0 22,392 C3H8/C4H10 28,578 21,904 air 0 22,4

For the base gas data, gas properties taken from the DVGW worksheet G 260 appendix 1 and sample values from the GASCALC computer program from e.on Ruhrgas AG have been used. The data are summarized in Table 4. Based on the technical characteristics of combustion of the base gases, the calorific ranges for the processed biogas were determined. Here, the calorific values for the calculations were assumed to be quantity-weighted averages and a + / - 2 percent band was placed around these (Tab. 5). In this way, in the following sections admixtures whose corresponding calorific values lie in this interval will

> WS,n in kWh/m³

Density ρ in kg/m³

Methane number (+/-2)

HS,n in kWh/m³

North Sea I 88,6 12,2 15,4 0,81 72 Holland II 82,9 10,2 12,8 0,83 86 Weser Ems L Gas 87,81 9,85 12,53 0,80 102

**HS,n in kWh/m³** *m n*, *v* **in m3/kmol** 

into natural gas grids."

with respect to the Wobbe index and calorific value.

Processed biogases with these methane levels are generally H gases.

79.05 Vol.-% nitrogen. All flow rates are standard flow rates.

Table 3. Numerical values used for the calculation

in Vol.-%

**3.1 Data on the base gases** 

Designation ϕMethane

be determined.

Table 4. Base gases

regulations need to be considered when granting a permit. The technical conditions for setting up tank installations are defined in TRB 801 No.25 "LPG storage tank facilities".

Commercial LPG consists of at least 95 percent by mass of propane and propene, whereby the propane content must predominate. The remainder may consist of ethane (C2H6), ethene (C2H4), butane (C4H10) and butene (C4H8) isomers. The classification for commercial propene, butane and butene is equivalent. Note also the degree of purity according to DIN 51 622 [DIN 1985]: Data on sulphur or sulphur compounds are listed here.

In DIN 51624 "automotive fuels - natural gas requirements and test methods" [8-15]upper limits for the propane/butane mole fractions in natural gas of 6% / 2% in the total mixture and a methane number > 70 are required. EASEE-gas CBP (EASEE, 2005) specifies a hydrocarbon dew point of -2 ° C at 1-70 bar. For the calculations shown below, a typical LPG composition of propane / butane, 95 / 5 is used.
